Liquid-contacting columns are provided for a wide variety of purposes and generally comprise so-called distributor plates which are horizontal, vertically spaced elements upon which the liquid is distributed in finely divided form, e.g. a thin film, droplets or streamlets, such that the plate is permeable to a rising fluid flow.
For example, such plates or stages may be used in liquid-gas contacting columns in which the liquid descends in counterflow to a rising gas for thermal exchange with the gas and/or material exchange therewith. In the latter case, for example, the liquid can be a solvent capable of extracting a component from the gas. In the former case, the liquid may be heated by the gas or the gas may be brought into thermal equilibrium with the liquid for other reasons.
Such columns have also been used for contacting two immiscible liquids with one another, e.g. for extraction or thermal exchange purposes, the more dense liquid descending from plate to plate while the less dense liquid rises through the column.
Since the efficiency of the exchange of the column is dependent upon the number of stages or levels, it is desirable to provide a column having a multiplicity of stages and levels and heretofore there has been a tendency to provide these levels as discrete plates formed with bubble caps or the like to distribute the liquid in thin-film or thin-layer, streamlet or droplet patterns to achieve a maximum surface area of exchange for a given volume on the particular plate.
There are, however, distributor stages for exchange columns in use in which each stage comprises a multiplicity of mutually parallel horizontal tubes or channels which are fed from manifold tubes or channels with the liquid and which have, along their respective lengths, spaced apart openings for distributing the liquid.
An important advantage of a tube distributor stage of this type is that it can operate in a closed piping system utilizing a liquid under an elevated pressure and to which an elevated flow velocity can be provided by this higher pressure.
Such stages can thus be operated under forced-flow principles with comparatively large throughputs for proportionately small flow cross sections, thereby providing a greater free cross section (in the column) for the rising phase, e.g. a gas or vapor phase.
This means that the column can operate with a reduced pressure drop for a given number of stages and a larger actual cross section for the vapor phase.
A tube distributor of the aforedescribed type has, however, the disadvantage that because of the relatively high discharge velocities of the liquid from the various holes or openings, only a limited number of comparatively small openings can be provided. When the distributor stage is of the trough-type, the number of run off locations can be made substantially larger, thereby increasing the uniformity of distribution of the liquid. Hence trough-type distributors have certain advantages over the tube-type distributors.
Trough-type distributors, however, also have certain disadvantages. For example, in practice it is found that conventional distributing troughs of such arrangements must be fed highly uniformly with the liquid so that the liquid is always at a predetermined depth which is uniform for all of the troughs.
This is an extremely costly and complicated system which has not found extensive application in practice.
It is also known to provide a distributor stage for an exchange column which comprises parallel horizontal channels in each of which a horizontal tube is provided. A lower portion of each tube is formed with a plurality of openings which communicate with the interior of the channel and these tubes are supplied centrally of their respective lengths with liquid from a common manifold.
Even with this system, varying levels of liquid in the channels create problems which cannot be satisfactorily dealt with economically with the earlier arrangement.